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About pH

Basic principle of the electrode
Potential analysis method used in the electrode is called a galvanic cell. Primary battery is a system, its role is to make the chemical energy into electricity. The battery voltage is called the electromotive force (EMF). The electromotive force (EMF) consists of two half cell. One and a half cells called measuring electrode, and its potential is associated with specific ion activity; Another one and a half in reference battery, often referred to as the reference electrode, it is general and measuring solution are interlinked, and connected to the measuring instrument.
For example, an electrode formed a thong in the salt solution of silver ions in a silver conductor is made, at the interface between conductor and solution, because the metal salt solution and two phase different activity of silver ion, it forms ions of charging process, and form a certain potential difference. Lose electronic silver ions into the solution. When there is no reverse charge the current stage, that is not current, a process that will eventually reach a balance. In this state of equilibrium voltage is called the half cell potential or existing in the electrode potential.
This (as above) is composed of metal and solution containing this metal ion electrode is called the first kind of electrode.
The measurement of this potential is relatively a potential has nothing to do with the composition of the salt solution of reference electrode. The reference electrode with independent potential is also known as the second electrode. For such electrodes, metal wires are covered a layer of the metal micro soluble salt (such as: Ag/AgCl), and inserts in electrolyte solution containing this kind of metal salt anions. The half cell potential or the size of the electrode potential depends on the activity of the anion.
The voltage between two electrodes follow the NERNST (NERNST formula:


Type: E - potential
E0 standard voltage, electrode
R - gas constant (8.31439 joules/Moore and ℃)
T - kelvin's absolute temperature (20 ℃ = 273 example: + 293 degrees kelvin)
F - ferrari brother constant (96493 coulomb/equivalent)
N - measured ion valence (silver = 1, h = 1)
AMe - ion activity
The standard hydrogen electrode is all potential measuring reference point. Standard hydrogen electrode is a platinum wire, use the method of electrolytic plating platinum chloride (coating), and surrounded by filling the hydrogen (fixed pressure 1013 hpa).
The electrodes immersed in 25 ℃ H3O + ion content is 1 mol/l in the solution, it forms all half cell potential measurement refers in the electrochemical potential or electrode potential. The hydrogen electrode as reference electrode is difficult in practice, then use the second electrode as the reference electrode. One of the most commonly used is the silver/silver chloride electrode. The electrode by dissolving the AgCl react to changes of chlorine ion concentration.
The reference electrode electrode potential through saturated KCL storage pool (such as: 3 mol/l KCL) to achieve a constant. In the form of a liquid or gel electrolyte solution through the diaphragm and connected test solution.
Using the electrode combination - silver electrode and Ag/AgCl reference electrode can measure of silver ions content in the film flushing fluid. Can also be silver electrode for platinum or gold electrode of REDOX potential measurement. For example, some kind of metal ion oxidation phase.
The most familiar is the most commonly used PH indicator electrode is glass electrode. It's a end on blown glass tube for pH sensitive glass membrane. Tube filling containing saturated AgCl 3 mol/l KCL buffer solution, the pH value of 7. Exists in glass membrane on the surface of the second reflecting the PH value of the potential difference in Ag/AgCl conduction system, such as the second electrode, the export.
The potential difference also follow the nernst formula:

E = 59.16 mv / 25 ℃ per pH
Type of R and F is constant, n as the valence, each ion has its fixed values. For hydrogen ions, n = 1.
"T" as a variable, temperature plays a significant role in the nernst formula. As temperatures rise, potential value will increase. For every 1 ℃ temperature change is big, will cause potential 0.2 mv/per pH changes. With the pH value, per 1 ℃ were pH 0.0033 per 1 pH changes.
That is to say: for about 20 ~ 30 ℃, and 7 ph measurement, do not need to compensate for temperature change; For temperature 30 ℃ > or < 20 ℃ and pH > 6 or 8 pH pH applications must compensate to temperature changes.
Figure 1: pH - potential - the relationship between the ion concentration
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 OH ions
11 12 13 14 October 9 8 7 6 5 4 3 2 1 0 H ion
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH
` + 414.4 • • • • • • • • • • • • • • • • • • • • • • • • +. 59.2 0-59.2,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 414.4 mv / 25 ℃